Integrated circuits are used for the measurement of optical absorption properties of fluids, such as gases or biological fluids, as function of wavelength, using integrated optical waveguides. The fluid being analyzed may for example be introduced in an integrated microfluidic channel that is arranged such as to allow an interaction with electromagnetic radiation propagating through an integrated waveguide structure, for example such structure in or on an integrated photonics substrate. These measurements may be used to obtain colorimetric information pertaining to a fluid sample. While few spectral data samples may be sufficient to characterize a target compound in the fluid sample in some applications, a high spectral resolution may be used to accurately characterize a fluid in many situations, e.g. to accurately determine a concentration of a target analyte. For example, while a transmission peak at a predetermined wavelength may be highly sensitive to the concentration of the target analyte in the sample, changes in absorption are often associated with wavelength shifts, such that a good spectral resolution may also be used in accurately determining the concentration.
Particularly, highly sensitive optical absorption measurements as function of wavelength, e.g. forming an absorption spectrum of sufficient spectral resolution for the intended purpose, may be used for detecting biological and/or chemical agents in the sample being analyzed, e.g. for the detection of target analytes such as proteins, antigens or antibodies. Applications of such measurements may include for example environmental monitoring, toxicology, medical diagnostics and gas sampling.
For example, a laser may be used as light source, e.g. a tunable laser or one or more lasers with corresponding predetermined laser emission spectra. The light emitted by the laser may be coupled into a waveguide, such as a slab waveguide or rib waveguide. The waveguide may have a section of a substantial length, which is arranged in close proximity to the material under test. The tested material may for example be introduced into a microfluidic channel that is arranged sufficiently close to this waveguide section such that an interaction of the light conducted in the waveguide can occur with the material in the microfluidic channel, e.g. an interaction of the evanescent field of the light.
For some tests, presence of light at different frequencies may be desired. However, the use of multiple lasers having different, predetermined spectra, the use of tuneable lasers or of broadband lasers can be expensive, and such sources may be difficult to integrate in an integrated optical device. Inexpensive wideband light sources are also known, such as light emitting diodes (LED). However, such inexpensive wide-band light sources may have a high etendue when coupled into an integrated waveguide. This high etendue may severely reduce the coupling efficiency.